Active matrix type liquid crystal display apparatus

ABSTRACT

A liquid crystal display apparatus includes a liquid crystal panel having polarizers, and a back light having a light source for illumination of the liquid crystal panel which has a characteristic of spectral transmittance required to satisfy the following equation, x&gt;y&gt;z, when a medium tone display voltage varies in a range between a minimum and maximum voltage for a Blue pixel, where “x” is a value of the transmittance in the panel at a wavelength corresponding to a longest wavelength in a range of wavelengths designated for blue light, “y” is a value of the transmittance in the panel at a wavelength corresponding to a maximum value of the intensity in a range of wavelengths designated for green light, and “z” is a value of the transmittance in the panel at a wavelength corresponding to a maximum value of the intensity in a range of wavelengths designated for red light.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 11/299,699, filedDec. 13, 2005, now U.S. Pat. No. 7,433,012, which is a continuation ofU.S. application Ser. No. 10/629,711, filed Jul. 30, 2003, now U.S. Pat.No. 7,209,211, which is a continuation of U.S. application Ser. No.09/572,375, filed May 18, 2000, now U.S. Pat. No. 6,621,538, which is acontinuation of U.S. application Ser. No. 08/740,008, filed Oct. 23,1996, now U.S. Pat. No. 6,137,560, the subject matter of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display apparatus,and, more particularly, to an active matrix liquid crystal displayapparatus.

Various examples of liquid crystal display apparatus are disclosed, forexample, in Japanese Patent Publication No. 63-21907 (1988), UP,WO91/10936 and Japanese Patent Application Laid-Open No. 6-222397(1994), in which a pair of comb electrodes are used to apply an electricfield to a liquid crystal in a direction parallel to the surface of asubstrate. However, in a display system of this type wherein thedirection of an electric field applied to the liquid crystal iscontrolled to be parallel with the surface of a substrate by usingactive elements (hereinafter referred to as a horizontal electric fieldtype), no consideration is given to the characteristic of the lightsource required to decrease the power consumption of the whole liquidcrystal display apparatus. Further, no consideration is given to theconfiguration of the liquid crystal display apparatus required tosuppress color shift in response to the application of a voltage theretoand to prevent a color defect s from occurring.

In the establishment of a horizontal electric field, opaque electrodesare provided in a display pixel portion in order to produce an electricfield substantially in parallel with the surface of the substrate. Ascompared with the prior art type of display panel wherein an electricfield is applied in a direction substantially vertical to the surface ofthe substrate by using a transparent electrode, the aperture ratio maybe deteriorated and the brightness under a bright state may be reduced.Accordingly, it is necessary to use a high-intensity light source in thehorizontal electric field producing type of display panel.

Because the display mode effective for a liquid crystal displayapparatus of the horizontal electric field type is a double refractionmode, the transmittance T can be generally expressed by the followingequation (1):

$\begin{matrix}{T = {{To}\;\sin^{2}2{\theta \cdot {\sin^{2}\left( \frac{{nd}\;\Delta\; n}{\lambda} \right)}}}} & (1)\end{matrix}$where, To designates a coefficient and is determined mainly by thetransmittance of the polarizer used in the liquid crystal panel, θdesignates the angle between an effective optical axis in the liquidcrystal layer and a transmittance axis for polarized light, d designatesthe thickness of the liquid crystal layer, Δn designates the anisotropyof the refractive index of the liquid crystal layer, and λ denotes thewavelength of light. Because the transmittance of the liquid crystaldisplay apparatus has essentially a maximum value at a certainwavelength, the liquid crystal display elements are colored. Onesolution to the above equation is a value which satisfies a conditionwherein the peak wavelength becomes equal to the maximum wavelength 555nm for luminous efficiency under a retardation of 0 order, that is,(πd·Δn/555)=π/2. In this case, the transmittance falls suddenly on theshort wavelength side of the peak wavelength, and it decreases graduallyon the long wavelength side. Therefore, the liquid crystal displayelements are colored yellow. As a result, it is required to use a lightsource with the color of a cold color family which represents acomplementary color to yellow. In other words, it is required to use alight source with a high color-temperature characteristic.

In general, a fluorescent lamp is used as a light source for a liquidcrystal display apparatus. Because the luminous efficiency of thefluorescent lamp in a short wavelength region is less than that in along wavelength region, the brightness may be reduced, and so a largeconsumption of power is required to obtain a high brightness. Since thenormal voltage of the battery must be maintained for a long time, forexample, in a note book type personal computer or personal digitalassistance, it is required to avoid any increase in the powerconsumption.

Now, the display operation of a liquid crystal display apparatus of thehorizontal electric field type can be obtained in the double refractionmode, and the transmittance T can be generally expressed by thefollowing equation (2):T=T ₀·sin²2θ·sin² [(π·d _(eff) ·Δn)/λ]  (2)where, T₀ designates a coefficient and is determined mainly by thetransmittance of the polarizer used in the liquid crystal panel, θdesignates the angle between an effective optical axis in the liquidcrystal layer and a transmittance axis for polarized light, d_(eff)designates the thickness of the liquid crystal layer, Δ denotes theanisotropy of the refractive index of the liquid crystal layer, and λdesignates the wavelength of light. Further, the product of d_(eff) andΔ is referred to as retardation. Here, the thickness d_(eff) of theliquid crystal layer is not the thickness of the whole liquid crystallayer, but the thickness of the liquid crystal layer in which thedirection of alignment is changed when a voltage is applied.

In general, the molecules of the liquid crystal in the vicinity of theboundary surface of a liquid crystal layer do not change the alignmentdirection due to the effect of anchoring at the boundary surface even ifa voltage is applied. Accordingly, when the thickness of the wholeliquid crystal layer sandwiched between the substrates equals d_(eff),d_(eff)<d_(LC) always is maintained between the thickness d_(LC) andd_(eff). It is estimated that the difference between d_(LC) and d_(eff)equals about 20 nm to 40nm.

As clearly seen from the above equation (2), the transmittance of theliquid crystal display panel takes a maximum value at a specificwavelength (peak wavelength). Therefore, the liquid crystal displayelement is easily colored, in other words, it is easy to beunnecessarily colored.

Generally, the liquid crystal panel is constructed so that the peakwavelength may become equal to the maximum wavelength 555 nm forluminous efficiency, that is, (πd·Δn/555)=π/2. At this time, the liquidcrystal display element is colored yellow, because the spectraltransmittance falls suddenly on the short wavelength side of the peakwavelength, and it decreases gradually on the long wavelength side.

The extent of coloring extremely changes with the application of avoltage to the liquid crystal. As the magnitude of the voltage valuechanges from the minimum voltage required for display to the medium tonedisplay voltage and then to the maximum voltage, the color tone isgradually changed. Therefore, the display state of colors is extremelydeteriorated.

Because the difference between the thickness of the liquid crystallayers appears as a change in the peak wavelength in the birefringencemode, the local and abnormal thickness of the liquid crystal displaylayer causes display defects, such as variations in the intensity and/orcolor tone, which are different from those in its surrounding area.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved liquidcrystal display apparatus, in which a low power consumption and a finedisplay characteristic are compatible with each other.

Another object of the present invention is to provide an improved liquidcrystal display apparatus which can suppress color shift caused by theapplication of a voltage and reduce the occurrence of a color defect dueto a local difference in thickness in the liquid crystal layer.

A liquid crystal display apparatus according to the present inventioncomprises a liquid crystal panel having a pair of substrates, aplurality of electrodes formed on at least one of said pair ofelectrodes and a liquid crystal layer sandwiched between said pair ofsubstrates, and a light source provided on one surface of said liquidcrystal panel. The light source has a luminous characteristic with thechromaticity of a warm color family and said liquid crystal panel has acharacteristic of spectral transmittance with the chromaticity of a coldcolor family. Thereby, the color of said light source can becompensated.

The warm color family includes colors with a reddish hue, such as yellowor orange, in contradistinction with “white” illuminated from thestandard illuminant C. The cold color family includes colors with abluish hue in contradistinction with “white” illuminated from thestandard illuminant C. While an illuminant with a color of the warmcolor family has a transmittance which is low at a shorter wavelength,for an illuminant with a color of the cold color family, thetransmittance is low at a longer wavelength. Therefore, by combiningthem, it becomes possible to transmit light almost uniformly in thevisible region. As a result, the display of the whole liquid crystaldisplay apparatus approaches “white”, as illuminated from the standardilluminant C.

The reason why the power consumption is reduced by using the presentinvention is as follows. The fluorescent lamp with a color of the warmcolor family tends to consume less electric power than one with a colorof the cold color family while obtaining the same intensity. In general,it is assumed that the power consumption of a fluorescent lamp with acolor temperature of 6000K is 1, the power consumption required toobtain the same intensity results in a 5% increase in a fluorescent lampwith a color temperature of 8000K and a 10% increase in one with atemperature of 10000K, but a 5% decrease in one with a temperature of4000K. For example, in order to compensate the color in a liquid crystaldisplay element colored in yellow, by using a fluorescent lamp with acolor temperature more than the 6770K of the white standard illuminantC, it is necessary to use an illuminant with a color temperature ofpreferably more than 10000K. For example, if an electric power of 2watts is consumed by using a fluorescent lamp with a color temperatureof 8700K in the liquid crystal display apparatus of the horizontalelectric field type, an electric power of 2.06 watts is consumed when afluorescent lamp with a color temperature of 10000K is used. However, ifthe fluorescent lamp with a color temperature of 6000K lower than thatof the white standard illuminant C is used, the power consumption is1.87 watts, and if one with 4000K is used, it becomes 1.79 watts.

The illuminant with a color of the warm color family may be made bychanging the kind of fluorescent materials being used and their mixingratio. A narrow band emission type fluorescent lamp can be made bymixing the materials selected from each of the following A, B and Cgroups. The A group has an emission peak in the range of 450 nm to 490nm, and includes the following materials:

-   -   3Ca₃(PO₄)₂.Ca(F,Cl)₂:3b³⁺, Sr₁₀(PO₄)₆C₁₂:Eu²⁺, (Sr,        Ca)₁₀(PO₄)₆C₁₂:Eu²⁺(Sr, Ca)₁₀(PO₄)₆C₁₂.nB₂O₃:Eu+²,        (Ba,Ca,Mg)₁₀(PO₄)₆C₁₂:Eu²⁺, Sr₂P₂OT:Sn²⁺, Ba₂P₂T:Ti⁴⁺,        2SrO.0.84P₂O₆.).16B₂O₃:Eu²⁺, MgWO₄, BaA₁₈O₁₃:Eu²⁺,        BaMg₂Al₁₆O₂₇Eu²⁺Mn²⁺, SrMgAl₁₀O₁₇:Eu²⁺        The B group has an emission peak in the range of 540 nm to 550        nm, and includes the following materials:    -   LaPO₄:Ce³⁺, Tb³⁺, LaO₃.0.2SiO₂.0.9P₂O₅:Ce³⁺, Tb³⁺, Y₂SiO₅:Ce³⁺,        Tb³⁺, CeMgAi₁₁O₁₉:Tb³⁺, CdMgB₅O₁₀:Ce³⁺, Tb³⁺        The C group has an emission peak in the vicinity of 610 nm, and        includes the following materials:    -   (Sr,Mg)₃(PO₄)₂:Sn²⁺, CaSiO₃:Pb²⁺, Mn²⁺, Y₂O₃:Eu³⁺, Y(P,V)O₄:Eu³⁺        By changing the mixing ratio, it becomes possible to control the        relative intensity of each of the emission regions, and thus        realize a fluorescent lamp with various color temperatures.        Further, by increasing the mixing ratio of the fluorescent        materials having an emission peak around 610 nm, it becomes        possible to make a fluorescent lamp with a lower color        temperature in the warm color family.

There are three methods to realize a liquid crystal display apparatus ofthe cold color family.

-   (1) A characteristic of the cold color family can be obtained by    positioning the maximum value of the transmittance in a short    wavelength area. The luminescence spectrum of the fluorescent    material corresponding to green resides in the range of 540 nm to    550 nm, and that corresponding to blue in the range of 450 nm to 490    nm. It is, therefore, possible to obtain a liquid crystal display    apparatus of the cold color family when the maximum luminescence    spectrum is less than 520 nm, that is, when d·Δn=0.26 in the    equation (1), because the blue color is emphasized in such a case.    Here, d denotes the thickness (d_(eff)) of the liquid crystal layer    which changes the direction of alignment when a voltage is applied.    The molecules of the liquid crystal in the vicinity of the boundary    surface of the liquid crystal layer does not change the direction of    alignment due to the effect of anchoring of the boundary surface    even when a voltage is applied. When the thickness of the liquid    crystal layer sandwiched between the substrates is d_(LC), the    thickness of the liquid crystal layer which changes the direction of    alignment when a voltage is applied is d_(eff), d_(eff)<d_(LC) and    the difference between d_(eff) and d_(LC) may be about from 300 nm    to 400 nm.-   (2) The liquid crystal display panel may be provided with a    birefringent film, which is set so as that the peak wavelength of    the spectrum transmittance in the liquid crystal display panel can    be within the short wavelength range of the visible light of 400 nm    to 520 nm, preferably 440 nm to 490 nm.-   (3) The liquid crystal display panel may be provided with a color    filter. The thickness of the liquid crystal layer at a portion where    red light can be transmitted is less than the thickness d_(LC) of    the liquid crystal layer at a portion where green light or blue    light can be transmitted.

The threshold voltage Ec in the liquid crystal display apparatus isexpressed by the following equation:

$\begin{matrix}{{Ec} = {\frac{\Pi}{d_{LC}}\sqrt{\frac{K_{2}}{ɛ\; o\;{\Delta ɛ}}}}} & (3)\end{matrix}$where, d_(LC) designates the thickness of the liquid crystal layer, K₂represents an elastic constant, Δ∈ designates the anisotropy of adielectric constant of the liquid crystal, and ∈ denotes a dielectricconstant for a vacuum. As d_(LC) is reduced, the threshold voltageshifts to a higher voltage. By setting the thickness of the liquidcrystal to be thin at a pixel portion where red is displayed, it becomespossible to shift red, that is, a voltage-transmittance characteristicin a long wavelength region to a higher voltage side. Thereby, thetransmittance at the long wavelength region for each voltage issuppressed, and thus it becomes possible to make a liquid crystaldisplay. apparatus in which the transmittance in the short wavelengthregion is larger. In order to suppress sufficiently the transmittance inthe high wavelength region and hold a color balance, it is preferablethat the change in thickness of the liquid crystal is suppressed withinthe range of 0.1 μm to 1 μm. For example, it is possible to reduced_(LC) by thickening the film at a portion of the color filter where redis displayed. It may be possible to thicken the film at a portion of thecolor filter where blue is displayed more than d_(LC) at portions lowhere red and green are displayed. Also, in this case, it is preferablethat the change in the thickness of the liquid crystal layer is withinthe range of 0.1 μm to 15 μm.

The illuminant used in accordance with the present invention has amaximum value of at least one intensity in each range from 400 nm to 500nm, from 500 nm to 600 nm and from 600 nm to 700 nm of said lightsource, and the liquid crystal panel has a characteristic of spectraltransmittance required to satisfy the relation, x>y>z, where x equals avalue of the transmittance at the wavelength which shows the maximumvalue of the intensity in the range from 400 nm to 500 nm, y denotes avalue of the transmittance at the wavelength which shows the maximumvalue of the intensity in the range from 500 nm to 600 nm, and z denotesa value of the transmittance at the wavelength which shows the maximumvalue of the intensity in the range from 600 nm to 700 nm. Thus, it ispossible to suppress the color shift caused by the change in the appliedvoltage and to provide a liquid crystal display apparatus having a finedisplay characteristic.

The reason why a fine display characteristic can be obtained will beexplained hereinafter.

As described above, the liquid crystal display apparatus is generallyoperated in a birefringent mode. Its transmittance is expressed in theequation (2). Accordingly, the liquid crystal display apparatus has aspectral transmittance which has its maximum value at a certainwavelength, suddenly decreases on the shorter wavelength side, andgradually decreases on the longer wavelength side. It is assumed thatthe peak wavelength is around 550 nm. The transmittance suddenlydecreases in the range of 400 nm to 500 nm, which is in a blue region.As the brightness of the liquid crystal panel increases, the dependenceof the transmittance on the wavelength becomes remarkable. Accordingly,this is the factor which causes the color shift according to a change inthe applied voltage.

When the thickness of the liquid crystal layer at a certain portion islocally different from other portions in the liquid crystal displayapparatus, the transmittance of blue at the portion remarkably changesand a color defect may occur. Accordingly, it should be noted that it isimportant to suppress any sudden decrease of the transmittance in theshort wavelength region of the peak wavelength or the blue region. Inorder to suppress a sudden decrease of the transmittance in the shortwavelength region, it is effective to shift the peak wavelength to theshort wavelength side by setting the wavelength λ to be shorter than 550nm under the condition of d_(eff)·Δn(λ)=λ/2.

The more the wavelength λ is spaced from the peak wavelength, the morethe extent of the decrease of the transmittance increases. It is,therefore, possible to suppress a sudden decrease of the transmittancein the short wavelength region by setting the peak wavelength to theshorter wavelength side. It is also important to suppress a suddendecrease of the transmittance at the wavelength of emission from theilluminant being used.

In general, a narrow band emission type fluorescent lamp is used for theilluminant of the liquid crystal display apparatus. Such a fluorescentlamp uses materials which have a luminescence peak at each spectrumregion of red (R), green (G) and blue (B).

The following group has an emission peak in the range of 450 nm to 490nm corresponding to blue, and includes the following materials:

-   -   3Ca₃(PO₄)₂.Ca(F,Cl)₂:3b³⁺, Sr₁₀(PO₄)₆C₁₂:Eu²⁺,        (Sr,Ca)₁₀(PO₄)₆C₁₂:Eu²⁺, (Sr, Ca)₁₀ (PO₄)₆C₁₂ nB₂O₃:Eu+²,        (Ba,Ca,Mg)₁₀(PO₄)₆C₁₂:Eu²⁺, Sr₂P₂O_(T):Sn²⁺,        Ba₂P_(2T):Ti⁴⁺2SrO.0.84P₂O₆.).16B₂O₃:Eu²⁺, MgWO₄, BaA₁₈O₁₃:Eu²⁺,        BaMg₂Al₁₆O₂₇:Eu²⁺Mn²⁺, SrMgAL₁₀O₁₇:Eu²⁺        The following group has an emission peak in the range of 540 nm        to 550 nm corresponding to green, and includes the following        materials:    -   LaPO₄:Ce³⁺, Tb³⁺, LaO₃0.2SiO₂.0.9P₂O₅:Ce³⁺, Tb³⁺, Y₂SiO₅:Ce³⁺,        Tb³⁺, CeMgAi₁₁O₁₉:Tb³⁺, CdMgB₅O₁₀:Ce³⁺, Tb³⁺        The following C group has an emission peak in the range of 610        nm to 630 nm corresponding to red, and includes the following        materials:    -   (Sr,Mg)₃(PO₄)₂:Sn²⁺, CaSiO₃:Pb²⁺, Mn²⁺, Y₂O₃:Eu³⁺, Y(P,V)O₄:Eu³⁺

The luminescence characteristic of the narrow band emission typefluorescent lamp made with fluorescent materials selected from each ofthe above groups is as follows. The spectrum corresponding to blue iswithin the range of 450 nm to 490 nm, the spectrum corresponding togreen is in the vicinity of 545 nm, and the spectrum corresponding tored is within the range of 610 nm to 630 nm.

Therefore, the characteristic of the spectrum transmittance which shouldbe taken into consideration in the liquid crystal panel using the abovenarrow band emission type fluorescent lamp is as follows. It should havethe range of 450 nm to 490 nm as a blue region, the range in thevicinity of 545 nm as a green region, and the range of 610 nm to 630 nmas a red region.

Accordingly, the most effective characteristic of the transmittance tosuppress color shift and/or color defects in the liquid crystal panelhas a maximum value in the wavelength region of 450 nm to 490 nm. Theretardation d_(eff)·Δn(A) should be set to be less than 0.245 μm (λ=490nm) to fit the peak of the transmittance to the above wavelength region.Further, it is necessary to use a liquid crystal material which has asmall anisotropy Δn of refractive index and a thin liquid crystal layerto reduce the retardation d_(eff)·Δn.

As described above, it is important to fit the characteristic of theluminescence at the short wavelength region of the illuminant to thepeak of the transmittance of the liquid crystal panel. Here, thespectral transmittance does not mean the spectral characteristic afterpassing through a color filter, etc., but refers to the characteristicof the transmittance of the liquid crystal panel itself.

While the peak wavelength of the transmittance may change a little byusing a certain color filter, it is possible to ignore its effect duringactual use. The most important point in accordance with the presentinvention resides in the relationship between the peak of the intensityof the illuminant and the transmittance of the liquid crystal panel.

The magnitude of the anisotropy Δn of the refractive index of the liquidcrystal changes according to temperature. If the temperature of theliquid crystal panel changes due to the environment of the place wherethe display apparatus is used, the set value of the retardationd_(eff)·Δn may change.

In a liquid crystal in which the anisotropy Δn of the refractive indexis relatively small, the change in the anisotropy Δn itself of itsrefractive index becomes small. In addition, if the thickness d_(eff) isalso small, the change in the product d_(eff)·Δn of the thickness andthe anisotropy becomes smaller. Accordingly, by using the above liquidcrystal, it is possible to obtain an expansion of the margin of thetemperature, and thus suppress any change in the retardation d_(eff)·Δn.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of the liquid crystal display apparatusaccording to the present invention.

FIG. 2 is a graph which shows a characteristic of the spectraltransmittance of a liquid crystal display panel of the horizontalelectric field type.

FIG. 3 is a diagram which shows the definition of a direction of rubbingand a direction of the axis of a polarizer.

FIGS. 4( a) and 4(b) are side-sectional views each showing one pixelportion of the liquid crystal display panel of the horizontal electricfield type.

FIGS. 4( c) and 4(d) are front views of the panels of FIGS. 4( a) and4(b), respectively, wherein active elements are not shown except a gateinsulating film 2.

FIG. 5 is a graph which shows color temperatures and chromaticitycoordinates.

FIG. 6 is a diagram which shows the configuration of the substrate of acolor filter.

FIGS. 7( a) and 7(b) each show an shows the emission characteristic ofan illuminant.

FIGS. 8( a) and 8(b) each show an the emission characteristic of anilluminant.

FIG. 9( a) shows the spectral transmittance of the liquid crystaldisplay panel when a drive voltage is applied.

FIG. 9( b) shows a luminescence spectrum obtained by using a lightsource.

FIG. 10 is a graph which shows chromaticity coordinates concerning theabove construction members.

FIG. 11A shows the spectral transmittance of the liquid crystal displaypanel when a drive voltage is applied.

FIG. 11B shows a luminescence spectrum obtained by using a light source.

FIG. 12 is a graph which shows chromaticity coordinates of the liquidcrystal display apparatus.

FIG. 13A shows a luminescence spectrum and FIG. 14B shows chromaticitycoordinates concerning the construction members.

FIG. 14A shows a luminescence spectrum and FIG. 14B shows chromaticitycoordinates concerning the construction members.

FIG. 15 shows a trail appearing on chromaticity coordinates.

FIG. 16 shows a voltage-transmittance characteristic of the liquidcrystal display apparatus.

FIG. 17 shows a voltage-transmittance characteristic of the liquidcrystal display apparatus.

FIG. 18 shows a voltage-transmittance characteristic of the liquidcrystal display apparatus.

FIG. 19 shows a trail appearing on the chromaticity coordinates.

FIG. 20 is a cross-sectional view of a liquid crystal display panel ofthe horizontal electric field type, FIG. 20( a) is a view taken alongline A-A′ and FIG. 20( b) is a view taken along line B-B′ in FIG. 20.

FIG. 21 is a cross-sectional view of a liquid crystal display panel ofthe horizontal electric field type, FIG. 21 (a) is a view taken alongline A-A′ and FIG. 21( b) is a view taken along line B-B′ in FIG. 21.

FIG. 22 is a diagram of a color filter according to an embodiment of thepresent invention, FIG. 22( a) is a view taken along line A-A′ and FIG.22( b) is a view taken along line B-B′ in FIG. 22.

FIG. 23 is a schematic view of a driving circuit for the liquid crystaldisplay apparatus.

FIG. 24 is a diagrammatic view of a color filter according to anembodiment of the present invention.

FIG. 25 shows an emission spectrum.

FIG. 26 shows an illustration of spectral transmittance.

FIG. 27 shows one example of an operating characteristic of a drivingcircuit for the liquid crystal display apparatus.

FIG. 28 shows an illustration of spectral transmittance.

FIG. 29 shows an illustration of color shift.

FIG. 30 shows an illustration of color shift.

FIG. 31 shows one example of an operating characteristic of a drivingcircuit for the liquid crystal display apparatus.

FIGS. 32 and 33 show the characteristics of the brightness to theapplied voltage in embodiments A and B by setting each of R, G and B asparameters.

FIGS. 34 and 35 show characteristics similar to those of FIGS. 31 and 32with regard to comparison examples C and D.

FIG. 36 shows a characteristic of transmittance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the liquid crystal display apparatus according to thepresent invention will be explained hereinafter with reference to theattached drawings.

Firstly, the configuration and the principle of operation of a liquidcrystal display element of the horizontal electric field type will beexplained with reference to FIG. 3, which illustrates the definition ofthe direction of rubbing and the direction of the axis of a polarizer.

In FIG. 3, reference numeral 1 designates a common electrode, 4designates a pixel electrode, 9 designates the direction of an electricfield, 10 denotes a longitudinal axis (optical axis) of a molecule ofthe liquid crystal, and 11 denotes the transmitting axis of polarizedlight. Further, ΦP designates an angle between the transmitting axis 11and a polarizer 8 (see FIG. 1), and ΦLC designates an angle between thedirection 9 of the electric field and the optical axis 10 in thevicinity of a boundary surface.

Because there are a pair of polarizers and a pair of boundary surfaces,respectively, an upper one and a lower one, the relevant angles areexpressed as ΦP1, ΦP2, ΦLC1, and ΦLC2, respectively, if necessary.Further, the longitudinal axis 10 of the molecule of liquid crystal isoriented in the same direction as that of rubbing due to an alignmentcontrol film.

Secondly, the configuration and the principle of operation of a liquidcrystal display panel of the horizontal electric field type will beexplained with reference to FIGS. 4( a) to 4(c).

FIGS. 4( a) and 4(b) are side-sectional views each showing one pixelportion of the liquid crystal display panel of the horizontal electricfield type, and FIGS. 4( c) and 4(d) are front views thereof. In thesefigures, active elements are not shown, except for a gate insulatingfilm 2. FIGS. 4( a) and 4(c) each show a state in which a voltage is notapplied. Stripes of electrodes 1,3 and 4 are formed inside a pair ofsubstrates 7, and alignment control films 5 are formed on thoseelectrodes and substrates. In addition, polarizers 8 are providedoutside the substrates 7, and the transmitting axes thereof are shown inFIG. 4( c). Although the liquid crystal composite is sandwiched betweenthe alignment control films 5, only the liquid crystal molecules 6 areshown in the figures. In this example, it is assumed that the dielectricanisotropy of the liquid crystal molecules is positive.

The molecules of liquid crystal are alignment-controlled according to arubbing direction 10 of the alignment control film 5 when a voltage isnot applied.

The angle ΦLC is controlled so as to satisfy the relation, 45°<|ΦLC|□90°in consideration of the dielectric positive anisotropy. In this example,the directions of alignment of the molecules of the liquid crystal onthe upper and lower boundary surfaces are parallel to each other,namely, ΦLC1=ΦLC2.

When a voltage with a predetermined polarity is applied, and thus anelectric field 9 is produced, the molecules of liquid crystal changetheir directions to align with the direction of the electric field 9, asshown in FIGS. 4( b) and 4(d). As a result, the transmittance of lightcan be controlled according to the magnitude of the applied voltage,with respect to the transmitting axis of the polarized light of thepolarizer 8, and thereby information can be displayed on the liquidcrystal display panel.

This operation may be normally performed even if the composition of theliquid crystal has a negative dielectric anisotropy. In this case, itshould be noted that the state of original alignment must be set to0°<|ΦLC|□45°.

Referring now to FIG. 1, there is shown a schematic diagram of theliquid crystal display apparatus according to the present invention. Thedisplay apparatus is provided with an illuminant in the form of an edgelight type back light unit having a light source 30, a wave guide 32, adiffuser 33 and a prism sheet 34. The light source 30 has a colortemperature of 5885K and the luminescence spectrum shown in FIG. 7( a).

FIG. 9( a) shows the spectral transmittance of this liquid crystaldisplay panel when a drive voltage is applied, and FIG. 9( b) shows aluminescence spectrum obtained by using the above light source. A powerof 1.8 Watts is consumed in the light source unit. Further, FIG. 10shows the chromaticity coordinates concerning the above construction.The liquid crystal display panel uses a color of the cold color family.Fine white balance can be obtained by combining the display panel havinga light source with a lower color temperature.

A nematic liquid crystal composition is inserted between the substrates,of which the anisotropy of the dielectric constant is positive, +9.0,and the anisotropy of the refractive index is 0.082 (589 nm, 20° C.).The gap d between cells equals 3.8 μm. As a result, d_(LC)·Δn equals0.31 μm, and d_(eff)·Δn equals 0.28 μm.

The liquid crystal display apparatus may also be provided with an edgelight type back-light unit using a cold cathode fluorescent lamp as alight source having a color temperature of 11000K and the luminescencespectrum shown in FIG. 8( b).

FIG. 11A shows the spectral transmittance of this liquid crystal displaypanel when a drive voltage is applied, and FIG. 11B shows a luminescencespectrum obtained by using the above light source. Further, FIG. 12shows chromaticity coordinates concerning the above construction. Incase of the combination of a yellowish liquid crystal display panel anda light source with a color of the cold color family, a power of 2 Wattsis consumed in the light source unit.

When the light source is changed to an edge light type back-light unitwith a color temperature of 5885K, a power of 1.8 Watts is consumed inthe light source unit. FIG. 13A shows a luminescence spectrum obtainedby using this light source, and FIG. 13B shows chromaticity coordinatesconcerning this construction. In accordance with this example, it ispossible to obtain a visually yellowish display apparatus.

A color filter 24 is provided on the substrate opposite to the substratehaving transistor elements, as shown in FIG. 6.

In one example, a nematic liquid crystal composition is inserted betweenthe substrates, of which the anisotropy of the dielectric constant ispositive, +7.3, and the anisotropy of the refractive index is 0.074 (589nm, 20° C.). The gap d between cells equals 3.2 μm in such a state thatspherical polymer beads are scattered and sandwiched between thesubstrates and the liquid crystal is sealed in. As a result, d·Δn equals0.24 μm. FIG. 14A shows the spectral transmittance of this liquidcrystal display panel when a drive voltage is applied, and FIG. 14Bshows chromaticity coordinates concerning the liquid crystal displayapparatus including a light source. The chromaticity coordinate underthe application of a drive voltage is positioned around standard lightsource C. A power of 1.8 Watts is consumed in the light source unit.According to this example, it is possible to obtain a liquid crystaldisplay apparatus of the horizontal electric field type, which issuitable for a color display.

In another example, a nematic liquid crystal composition is insertedbetween the substrates, of which the anisotropy of the dielectricconstant is positive, +9.0, and the anisotropy of the refractive indexis 0.082 (589nm, 20° C.). The gap d between cells equals 3.7 μm in sucha state that spherical polymer beads are scattered and sandwichedbetween the substrates and the liquid crystal is sealed in. As a result,d_(LC)·Δn equals 0.30 μm, and d_(eff)·Δn equals about 0.27 μm. A phaseplate is attached between the upper substrate and the polarizer, so thatthe angle ΦF1 of the optical axis may become parallel with the uppersubstrate, in other words, QF1=ΦP1=75°. The phase plate is made of polycarbonate and has a retardation of 595 nm (550 nm). The liquid crystaldisplay panel is provided with an edge light type back-light unit usinga cold cathode fluorescent lamp as a light source. The light source shas a color temperature of 4348K and the luminescence spectrum shown inFIG. 8( a).

FIG. 15 shows a trail appearing on the chromaticity coordinates when thevoltage of the liquid crystal display apparatus is switched from ON toOFF. The trail approaches a light source C. A power of 1.7 Watts isconsumed in the light source unit.

A striped color filter 24 with three colors, R, G, B, is provided on thesubstrate 7 opposite to the substrate having transistor elements, asshown in FIG. 6. A surface flattening protection film 25 is provided onthe color filter 24, and the alignment film 5 is formed on theprotection film 25. A phase plate is attached between the substrate andthe polarizer, so that the angle ΦF1 of the optical axis may becomeorthogonal to the upper polarizer, in other words, ΦF1=ΦP2=−15°. Thephase plate is made of poly carbonate and has a retardation 349 of nm(550 nm). The liquid crystal display panel is provided with an edgelight type back-light unit using a cold cathode fluorescent lamp as alight source. The light source has a color temperature of 4703K and aluminescence spectrum as shown in FIG. 7( b). The chromaticitycoordinates when a drive voltage of the liquid crystal display apparatusis applied is close to the light source C. A power of 1.75 Watts isconsumed in the light source unit.

The thickness of the film of the color filter was about 2 μm at the Band G pixels, and about 2.5 μm at the R pixels. The difference betweenthese thicknesses remains as a level difference of about 0.3 μm afterspin-coating the flat film. The level difference represents thedifference of thickness between the liquid crystal layers. The edgelight type back light unit, which was used for the liquid crystaldisplay panel, included a cold cathode fluorescent lamp with a colortemperature of 4703K. FIG. 16 shows a voltage-transmittancecharacteristic of this liquid crystal display apparatus at wavelengthsof 615 nm, 545 nm and 465 nm, that is, the voltage-transmittancecharacteristic corresponding to each of the R, G and B pixels. It isseen from FIG. 16 that the characteristic of the transmittance of the Rpixels is shifted to the high voltage side. Accordingly, when the drivevoltage for the liquid crystal panel was applied, its transmittance hada characteristic in which red is suppressed. The white balance was finewhen the drive voltage was applied, and an electric power of 1.75 Wattswas consumed in the illuminant unit.

In another example, the thickness of the film of the color filter wasabout 2 μm at the G and R pixels and about 1.5 μm at the B pixels. Thethickness of the liquid crystal layer was about 3.8 μm at the G and Rpixels, and about 4.1 μm at the B pixels. An edge light type back lightunit, which was used for the liquid crystal display panel, included acold cathode fluorescent lamp with a color temperature of 4703K. FIG. 17shows a voltage-transmittance characteristic of this liquid crystaldisplay apparatus at wavelengths of 615 nm, 545 nm and 465 nm, that is,a voltage-transmittance characteristic corresponding to each of the R, Gand B pixels. It is seen from FIG. 17 that the characteristic of thetransmittance of the B pixels is shifted to the low voltage side.Accordingly, when the drive voltage for the liquid crystal panel wasapplied, its transmittance had a characteristic in which blue isemphasized. The white balance was fine when the drive voltage wasapplied, and an electric power 1.75 Watts was consumed in the illuminantunit.

In a further example, the thickness of the film of the color filter wasabout 2 μm at the G pixels, about 1.5 μm at the B pixels and about 2.5μm at the R pixels. The thickness of the liquid crystal layer was about4.2 μm at the G pixels, about 3.9 μm at the R pixels, and about 3.9 μmat the B pixels. An edge light type back light unit, which was used forthe liquid crystal display panel, included a cold cathode fluorescentlamp with a color temperature of 4348K. FIG. 18 shows avoltage-transmittance characteristic of this liquid crystal displayapparatus at wavelengths of 615 nm, 545 nm and 465 nm, that is, thevoltage-transmittance characteristic corresponding to each of the R, Gand B pixels. It is understood from FIG. 18 that the characteristic ofthe transmittance of the B pixels is shifted to the low voltage side,and the characteristic of the transmittance of the R pixels is shiftedto the high voltage side. The white balance was fine when the drivevoltage was applied, and an electric power 1.7 Watts was consumed in theilluminant unit.

In a still further example, the thickness of the film of the colorfilter was about 2 μm at the G and R pixels, and about 1.5 μm at the Bpixels. The thickness of the liquid crystal layer was about 4.5 μm atthe G and R pixels, and about 4.2 μm at the B pixels. A phase differencefilm made of poly-carbonate, which has a retardation of 997 nm (550 nm),was inserted between the upper substrate and the polarizer, and it wasattached so that the angle ΦF1 of its delay-phase axis was parallel withthe upper polarizer, that is, ΦF1=ΦP1=75°. An edge light type back lightunit, which was used for the liquid crystal display panel, included acold cathode fluorescent lamp with a color temperature of 4348K. FIG. 19shows a trail appearing on the chromaticity coordinates. It is seen fromFIG. 19 that the trail approaches the standard illuminant C as a voltageis applied. The white balance was fine when the drive voltage wasapplied, and the electric power 1.70 watts was consumed in theilluminant unit.

FIGS. 20 and 21 show two different kinds of liquid crystal display panelof the horizontal electric field type. These figures each show a frontview seen from a direction perpendicular to the surface of thesubstrate, while FIGS. 20( a) and 21 (a) show side-sectional views takenalong the line A-A′ and FIGS. 20( b) and 21 (b) show side-sectionalviews taken along the line B-B′ in FIGS. 20 and 21, respectively. Aglass substrate is not shown.

In these figures, reference numeral 14 designates a thin filmtransistor, which has pixel electrodes (source electrodes) 4, signalelectrodes (drain electrodes) 3, a scanning electrode (gate electrode)12, and amorphous silicon 13. A common electrode 1 and the scanningelectrode 12 are formed by patterning the same metal layer formed on theglass substrate. The signal electrodes 3 and the pixel electrodes 4 areformed by patterning the same metal layer formed on a gate insulatinglayer 2. A load capacitance 16 is formed by allowing the insulatinglayer 2 to sandwich between the pixel electrodes 4 and the commonelectrode 1.

In FIG. 20, the pixel electrodes 4 are arranged between two commonelectrodes 1. An alignment control film 5 is provided directly on thegate insulating layer 2, which has also the function of asurface-flattening film. In this case, the pitch between the pixels is69 μm in a horizontal direction and 207 μm in a vertical direction.

The width of each electrode is determined as follows. In the electrodesused as a wiring electrode bridging a plurality of pixels, that is, thescanning electrode 12, the signal electrode 3 and the wiring portion(parallel with the scanning electrode, and in a horizontal direction inFIG. 20) of the common electrode 1, the width of those electrodes areset to be, for example, 14 μm to avoid a wire defect.

The width of the pixel electrode 4 formed independently for each pixeland the longitudinally extending portion of the common electrode 1 arerespectively set to be 9 μm. The common electrode 1 and the signalelectrode 3 are partially superposed on each other (by 1 μm) through theinsulating layer. Thereby, it becomes unnecessary to provide a blackmatrix in a direction parallel with the signal electrode 3. Accordingly,there is provided only a black matrix 22 which can shield the light inthe direction of the scanning electrode. In addition, a color filter 24is provided only on the surface of one substrate.

While the black matrix 22 is provided on the substrate in which theelectrodes are formed, it may be possible to provide the black matrix onthe opposing substrate. These electrodes can be formed in a conventionalway.

In the example of FIG. 21, the common electrodes 1 and the pixelelectrodes 4 are formed like a comb, in which two pixel electrodes 4 arearranged between three common electrodes 1. The pitch between the pixelsis 100 μm in a horizontal scanning direction and 300 μm in a verticaldirection. The insulating layer is provided on the portion where thecommon electrodes 1 and the signal electrodes 3 are superposed. Thethickness of the insulating layer is 2 μm.

Further, a surface-flattening insulating layer 27 is provided betweenthe alignment control film 5 and gate insulating layer 2. The materialfor the surface-flattening insulating layer 27 is SiO2 or SiN, the sameas the gate insulating layer 2. However, it may be possible to use othersuitable materials.

The width of each electrode is determined as follows.

In the electrodes used as a wiring electrode bridging between aplurality of pixels, that is, the scanning electrode 12, the signalelectrode 3 and the wiring portion (parallel with the scanningelectrode, and in a horizontal direction in FIG. 20) of the commonelectrode 1, the widths of those electrodes are set to be 10 μm, 8 μmand 8 μm, respectively, to avoid a wire defect.

The width of the pixel electrode 4 formed independently for each pixeland the longitudinally extending portion of the common electrode 1 areset to be 5 μm and 6 μm, respectively.

Because the width of the electrode is narrow in this example, thepossibility of breaks is increased due to the mixing of foreignparticles. In FIG. 21, a black matrix 22 is provided on the opposingsubstrate, along with the color filter 24, as shown in FIG. 22.Reference numeral 25 designates a protecting and surface-flatteninglayer. It is also possible to provide a color filter 24 on the opposedsubstrate or the substrate in which electrodes are formed. Theseelectrodes can be formed in a conventional way.

FIG. 23 shows one example of a driving circuit for the liquid crystaldisplay apparatus. In the driving circuit, a driving LSI is connected tothe active matrix type liquid crystal display panel 23. Scanning linedriving circuits 20, signal line driving circuits 21 and common linedriving circuits 26 are provided on a TFT substrate on which a pluralityof electrodes are mounted.

A scanning signal voltage, an image signal voltage and a timing signalare supplied from a power circuit (not shown) and a controller 19, andthen the display operation responsive to the active matrix drive isstarted.

Embodiments of the present invention will be explained hereinafter.

In FIG. 24, reference numeral 7 designates two substrates made of glassplates having a thickness of 1.1 mm. A thin film transistor is formed onone of the substrates (lower substrate in FIG. 24), and then aninsulating layer 2 and an alignment film 5 are formed on the surfacethereof. In this embodiment, polyimide is used for the alignment film,and a rubbing-processing is performed to align the liquid crystal. Analignment film is also formed on the other substrate (upper substrate inFIG. 24) and then rubbing-processing is performed. The directions of therubbing at the upper and lower substrates are in parallel with eachother and at an angle of 75° with respect to the direction of theapplied voltage, that is, ΦLC1=ΦLC2=75°.

A nemuatic liquid crystal composition is inserted between the substrates7, of which the anisotropy of the dielectric constant is positive,+12.0, and the anisotropy of the refractive index is 0. 079 (589 nm, 20°C.). The gap d between cells equals 3.02 μm due to the fact thatspherical polymer beads are scattered and sandwiched between thesubstrates and the liquid crystal is sealed in. As a result, thethickness of the whole liquid crystal layer d_(LC) becomes equal to thegap d (3.02 μm). The value of d_(LC)·Δn (589 nm) equals 0.239 μm, andfrom the wavelength dependence characteristic of the anisotropy of therefractive index, d_(LC)·Δn (490 nm) equals 0.244 μm. As a result,d_(eff)·Δn (490 nm) equals about 0.22 μm.

The pair of substrates 7 are sandwiched by two polarizers 8. Thepolarization axis of one substrate is set to satisfy ΦP1=75°, and thepolarization axis of the other substrate is set to satisfy ΦP2=−15°.Thereby, the liquid crystal display panel 23 shown in FIG. 24 isobtained.

As shown in FIG. 24, a back-light unit, which is provided as anilluminant for transmitting light to the liquid crystal display panel23, comprises a fluorescent lamp 30, a light cover 31, a guide 32 and apolarizer 33, and has a color temperature of 5885K.

It may be possible to from the back-light unit by using a plurality offluorescent lamps, and preferably, to provide a prism sheet between thepolarizer 33 and the lower substrate 8.

In order to obtain a display closest to the achromatic color from thecharacteristics of the color of the liquid crystal display panel 23itself, except for the color filter, the color temperature of theilluminant is determined. Its color temperature is 5885K.

The spectrum characteristic of the back-light is shown in FIG. 25, andthe characteristic of the spectral transmittance in the light state ofthe liquid crystal display panel 23, except for the color filter, isshown in FIG. 26. In this embodiment, the dependence of the brightnessof the liquid crystal display apparatus on an applied voltage is shownin FIG. 27.

As seen from FIG. 26, the color shift due to the intensity control issufficiently suppressed in this embodiment.

COMPARISON EXAMPLE 1

A nematic liquid crystal composition is inserted between the substrates,of which the anisotropy of the dielectric constant is positive, +9.0,and the anisotropy of the refractive index is 0.082 (589 nm, 20° C.).The gap d, between cells equals 3.83 μm.

In this comparison example, d_(LC)·Δn (589 nm) equals 0.310 μm, andd_(LC)·Δn (490 nm) equals 0.321 μm. As a result, d_(eff)·Δn (490 nm)equals about 0.30 μm. This value is out of the present invention.

This liquid crystal display panel is provided with an edge light typeback-light unit using a cold cathode fluorescent lamp as a light source.The light source has a color temperature 6818K. The characteristic ofthe spectral transmittance in a light state of the liquid crystaldisplay apparatus without the color filter is as shown in FIG. 28, inwhich the transmittance in the short wavelength region is remarkablydecreased. As a result, a trail appears on the chromaticity coordinatesuntil a voltage of the liquid crystal display apparatus is switched fromOFF (a dark state) to ON (a light state), as shown in FIG. 29, in whichthe color is shifted and the liquid crystal display panel itself iscolored.

As seen from the comparison example 1, the color is shifted as the darkstate is shifted into the light state in the liquid crystal displayapparatus using the liquid crystal display panel in which thetransmittance at the short wavelength is reduced. According to thiscomparison example, it is difficult to suppress the color shift in thecolor display and the coloring in the black and white display, and thusthe quality of the displayed image essentially deteriorates.

COMPARISON EXAMPLE 2

In the comparison example 2, a nematic liquid crystal composition isinserted between the substrates, of which the anisotropy of thedielectric constant is positive, +9.0, and the anisotropy of therefractive index is 0.082 (589 nm, 20° C.). The gap d between cellsequals 4.26 μm.

In this comparison example, d_(LC)·Δn (589 nm) equals 0.345 μm, andd_(LC)·Δn (490 nm) equals 0.357 μm. As a result, d_(eff)·Δn (490 nm)becomes equal to about 0.33 μm. This value is also outside of thepresent invention. It is understood that the transmittance for bluelight is reduced.

This liquid crystal display panel is provided with an edge light typeback-light unit using a cold cathode fluorescent lamp as a light source.The light source has a color temperature 6818K. A trail appears on thechromaticity coordinates until a voltage of the liquid crystal displayapparatus is switched from OFF (a dark state) to ON (a light state) asshown in FIG. 30. As seen from the comparison example 2, the color isshifted to a yellowish color, as the dark state is shifted into thelight state. Also, according to this comparison example, it is difficultto improve the quality of the displayed image.

The change in the characteristics caused by the local change in thethickness of the liquid crystal will be explained with reference tovarious embodiments and comparison examples.

Embodiment A

The liquid crystal display apparatus has two substrates, one of whichhas a color filter with B, G and R pixels on its surface. A nemnaticliquid crystal composition is inserted between the substrates, of whichthe anisotropy of the dielectric constant is positive, +12.0, and theanistropy of the refractive index is 0.079 (589 nm, 20° C.). The gap dbetween cells is formed by scattering spherical polymer beads andsandwiching them between the substrates. The gap is adjusted to d=2.87μm by selecting the radius of the beads.

In this comparison example, d_(LC)·Δn (589 nm) equals 0.227 μm andd_(LC)·Δn (490 nm) equals 0.232 μm. As a result, d_(eff)·Δn (490 nm)equals about 0.21 μm. This value is out of the present invention.

The liquid crystal display panel is provided with a back-light unit as alight source which has a color temperature of 6818K.

Embodiment B

The liquid crystal display apparatus has two substrates, one of whichhas a color filter with B, G and R pixels on its surface. A nematicliquid crystal composition is inserted between the substrates, of whichthe anisotropy of the dielectric constant is positive, +12.0, and theanisotropy of the refractive index is 0.079 (589 nm, 20° C.). The gap dbetween cells is formed by scattering spherical polymer beads andsandwiching them between the substrates. The gap is adjusted to d=3.17μm by selecting a radius of the beads which is different from that inembodiment A.

In this embodiment B, d_(LC)·Δn (589 nm) equals 0.250 μm and d_(LC)·Δn(490 nm) equals 0.256 μm. As a result, d_(eff)Δn (490 nm) equals about0.23 μm.

The liquid crystal display panel is provided with a back-light unit as alight source which has a color temperature of 4703K.

COMPARISON EXAMPLE C

The liquid crystal display apparatus has two substrates, one of whichhas a color filter with B, G and R pixels on its surface. A nematicliquid crystal composition is inserted between the substrates, of whichthe anisotropy of the dielectric constant is positive, +9.0, and theanisotropy of the refractive index is 0.082 (589 nm, 20° C.). The gap d,between cells is adjusted to d=3.83 μm.

In this comparison example, d_(LC)·Δn (589 nm) equals 0.314 μm, andd_(LC)·Δn (490 nm) equals 0.321 μm. As a result, d_(eff)·Δn (490 nm)equals about 0.30 μm. This value is out of the present invention.

The liquid crystal display panel is provided with a back-light unit as alight source which has a color temperature of 6818K.

COMPARISON EXAMPLE D

The liquid crystal display apparatus has two substrates, one of whichhas a color filter with B, G and R pixels on its surface. A nematicliquid crystal composition is inserted between the substrates, of whichthe anisotropy of the dielectric constant is positive, +9.0, and theanisotropy of the refractive index is 0.082 (589 nm, 20° C.). The gap dbetween cells is adjusted to d=4.26 μm.

In this comparison example, d_(LC)·Δn (589 nm) equals 0.349 μm, andd_(LC)·Δn (490 nm) equals 0.357 μm. As a result, d_(eff)·Δn (490 nm)equals about 0.33 μm. This value is also out of the present invention.

The liquid crystal display panel in the comparison example D is providedwith a back-light unit as a light source which has a color temperatureof 6818K.

As should be clearly understand from the above description, there is adifference of 10% in the gap between the embodiments A and B, andbetween the comparison examples C and D. Accordingly, it is possible toestimate the color shift caused by the change in the thickness d_(eff)(□d) of the liquid crystal layer, that is, the gap margin.

FIG. 31 shows the characteristic of the color difference ΔEuv* inrelation to the applied voltage in the embodiments A and B, and thecomparison examples C and D. The characteristic of the color differencecan be obtained by using the color difference equation of the main colorfamily concerning L*u*v* proposed by CIE in 1976.

In general, the value of the color difference ΔEuv* allowable in thesame liquid crystal display panel is around 3 in such a liquid crystaldisplay apparatus.

Referring now to FIG. 31, as clearly understood from the characteristicshown by the solid line of this figure, even if there is a difference of10% in the gap between the embodiments A and B in the same displaypanel, the value of the color difference ΔEuv* is held to less than 2.Accordingly, a color defect is not evident in this case.

While, as clearly understood from the characteristic shown by the dottedline of FIG. 31, if there is a difference of 10% in the gap between thecomparison examples C and D in the same display panel, a large colordifference ΔEuv* appears in response to the applied voltage. Therefore,a remarkable color defect may be expected in this case.

In the embodiments of the present invention, it is understood that evenif there is a difference of 10% in the gap between the embodiments A andB in the same display panel, a color defect is not evident in this case,and it is possible to obtain a sufficient margin for the change in thegap.

The reason for the occurrence of a color difference in the embodimentsof the present invention and the comparison examples will be explainedfrom the view point of the difference in the passing characteristic forthe colors, R, G and B.

FIGS. 32 and 33 show the characteristics of the brightness in relationto the applied voltage in the embodiment A and B by setting each of thecolors R, G and B as parameters. Further, FIGS. 34 and 35 showcharacteristics similar to FIGS. 31 and 32 with regard to the comparisonexamples C and D, where the value of the wavelength of each color wasmeasured by using the back-light with the luminescence characteristicsshown in FIG. 25. The value of the wavelength of B (blue) was set to themiddle value, 465 nm, of the spectra in a blue portion.

The following fact is clarified from these figures.

In the embodiments of the present invention shown in FIGS. 32 and 33,the tendency of the change in the characteristic of each color is thesame until the display is switched from a dark state to a light state,and the contribution of a color to the brightness is almost equal ineach color. Accordingly, the color shift is not evident in theseembodiments. In the comparison examples, the tendency (shown by a solidline) of the change in the characteristic of blue is different fromthose of red and green. As the applied voltage increases, thecontribution of blue to the brightness decreases. Accordingly, in theseexamples, as the brightness increases, the component of blue is reduced.As a result, a yellowish display appears, and thus the color is shifted.

In FIG. 36, the passing ratio of each wavelength in the light display isexpressed in terms of the brightness by setting the retardationd_(eff)·Δn (μm) as a parameter. As seen from FIG. 36, the brightness inthe short wavelength region (blue region) less than 500 nm extremelychanges and is remarkably reduced by a small change in the retardationd_(eff)·Δn.

It is important to maintain the relationship of the transmittancebetween the three wavelengths of the colors R, G and B to apredetermined state.

The predetermined state refers to a state wherein transmittance in thewavelength of the longest wave among the spectra corresponding to blueof the emission spectra of the back-light is always larger than that inthe wavelengths 545 nm (green) and 630 nm (red).

Accordingly, the present invention must satisfy the condition that theabove relationship is always maintained.

1. A liquid crystal display apparatus comprising: a liquid crystal panelincluding a pair of polarizers; and a back light having a light sourceprovide at a back side of said liquid crystal panel for illuminationthereof; wherein said liquid crystal panel is an active matrix typeliquid crystal panel enabling display in a double refraction mode, andhas a characteristic of spectral transmittance required to satisfy thefollowing equation, x>y>z, when a medium tone display voltage varies ina range between a minimum voltage and a maximum voltage for a Bluepixel, where: “x” is a value of the transmittance in said liquid crystalpanel at a wavelength which corresponds to a longest wavelength in arange of wavelengths designated for blue light illuminated from saidlight source; “y” is a value of the transmittance in said liquid crystalpanel at a wavelength which corresponds to a maximum value of theintensity in a range of wavelengths designated for green lightilluminated from said light source; and “z” is a value of thetransmittance in said liquid crystal panel at a wavelength whichcorresponds to a maximum value of the intensity in a range ofwavelengths designated for red light illuminated from said light source.2. A liquid crystal display apparatus comprising: a light crystal panelincluding a pair of polarizers; and a back light having a light sourceprovided at a back side of said liquid crystal panel for illuminationthereof; wherein said liquid crystal panel is an active matrix typeliquid crystal panel enabling display in a double refraction mode, andhas a characteristic of spectral transmittance required to satisfy thefollowing equation, x>y>z, when a medium tone display voltage, whichvaries in the range of overlapped voltages among medium tone displayvoltages for each pixel of Red, Green and Blue pixels, is applied from asignal line drive circuit to a signal electrode for each pixel of theRed, Green and Blue pixels, where: “x” is a value of the transmittancein said liquid crystal panel at a wavelength which corresponds to alongest wavelength in a range of wavelengths designated for blue lightilluminated from said light source; “y” is a value of the transmittancein said liquid crystal panel at a wavelength which corresponds to amaximum value of intensity in a range of wavelengths designated forgreen light illuminated from said light source; and “z” is a value ofthe transmittance in said liquid crystal panel at a wavelength whichcorresponds to a maximum value of intensity in a range of wavelengthsdesignated for red illuminated from said light source.
 3. A liquidcrystal display apparatus comprising: a liquid crystal panel including apair of polarizers; and a back light having a light source provided at aback side of said liquid crystal panel for illumination thereof; whereinsaid liquid crystal panel is an active matrix type liquid crystal panelenabling display in a double refraction mode, and has a characteristicof spectral transmittance required to satisfy the following equation,x>y>z, when a medium tone display voltage, which varies from a minimummedium tone display voltage to a maximum medium tone display voltage, isapplied for each pixel of Red, Green and Blue pixels, where: “x” is avalue of the transmittance in said liquid crystal panel at a wavelengthwhich corresponds to a longest wavelength in a range of wavelengthsdesignated for blue light illuminated from said light source; “y” is avalue of the transmittance in said liquid crystal panel at a wavelengthwhich corresponds to a maximum value of intensity in a range ofwavelengths designated for green light illuminated from said lightsource; and “z” is a value of the transmittance in said liquid crystalpanel at a wavelength which corresponds to a maximum value of intensityin a range of wavelengths designated for red light illuminated from saidlight source.